It has been well known for a long time that oral administration of some viable bacteria or lactic acid bacteria to human beings and animals improve intestinal function and widely practiced to orally administer such viable microorganism for maintenance and promotion of health, for instance by adding these microorganisms to foodstuffs or feedstuffs. The various kinds of microorganisms are used for that purpose and various kinds of products containing them are known, for instance powder, granule, gel, liquid and so on. Recently, medical preparations, foodstuffs and feedstuffs containing lactic acid bacteria or bifidobacteria have held public attention.
It is, however, very difficult to preserve such product containing viable microorganism cells for a long time without decreasing cell survival rate, because the viability of these bacteria in this product decreases when environmental conditions are changed during storage. However, when such microorganism cells are preserved under the condition of lowered environmental moisture microorganism cells turn to resting cells and it is possible to store the microorganism cells for a long period.
Thus, cultivated and collected microorganism cells have hitherto been washed, added with a cryoprotectant, freeze-dried or vacuum-dried, pulverized, sifted out, added with starch or lactose as excipient and added with sugar or acidulant to obtain a powdered product. Such powdered products are often reprocessed after addition of suitable substances into granular or other block-type products. In the conventional method, it is important to keep moisture content of the product as low as possible during all the processing steps. However, it is not always easy to strictly keep such condition. Resting cells of microorganism are activated by the addition of water to dried microorganism cells in the step of processing or absorbing atmospheric moisture by the dried product for the period of storage. In this case, however, as the condition of survival for microorganism cells is extremely unsuitable, it is impossible to achieve sufficient viability of the microorganism cells.
It has been known that viability of the product containing dried microorganism cells during the storage period is effected by the presence of atmospheric oxygen, since free radicals are induced therein.
Therefore, it has been disclosed to suspend such dried viable microorganism cells in oil or fat for preventing permeation of environmental moisture (Japanese Unexamined Patent Application Gazette 2908/1981), to coat powder containing dried viable bifidobacteria with fat or oil for preventing permeation of environmental moisture and atmospheric oxygen (Japanese Unexamined Patent Application Gazette 33543/1982) and to pelletize a mixture of powder containing dried viable bifidobacteria with oil or fat for preventing permeation of environmental moisture and atmospheric oxygen which is used as a material to prepare various confections (Japanese Unexamined Patent Application Gazette 32221/1982).
According to the above prior arts, however, dried viable microorganism cells are only mixed with fat or oil or only suspended in fat or oil. Thus, powder particles of the prior art are not always completely coated with fat or oil and consequently the products prepared by the above prior methods do not achieve sufficient viability.
Futhermore, it has also been known to mix dried viable bifidobacteria with sufficiently dried starch in order to extremely decrease the water content thereof to prepare pelletized confections (Japanese Unexamined Patent Application Gazette 4976/1984). According to this prior method, however, it might be possible to decrease the water content in the product just after manufacturing thereof but it is almost impossible to keep the product from absorbing environmental moisture and atmospheric oxygen as it is. The product must be contained in a hermetically sealed container in which air is replaced with nitrogen. It goes without saying that such additional step and cost of additional material will occupy a considerable portion of the whole cost of the product.
The inventors have tried to overcome or improve the defects in this technical field to find a way for granulating dried viable microorganism cells without using any water and to produce a granule having a stratified structure like an onion and excellent viability.